Polyphenylene is a polymer comprised of repeating phenyl groups. Polyphenylene has a number of desirable properties, including good near infrared transmission, low dielectric constant, low moisture uptake, thermal and environmental stability, and ease of pattern fabrication using lithographic techniques. Polyphenylene has the potential for a variety of uses in the microelectronics industry, including use as a planarizing material, as an insulator, as an encapsulator, as an interlevel dielectric in a multichip module, and as an optical waveguide, among others.
Various processes for synthesizing polyphenylene have been suggested. One such process is the direct polymerization of benzene, an oxidative cationic polymerization requiring large quantities of cupric chloride (CuCl.sub.2) disclosed by Kovacic et al., J. Polym. Sci. 47, 448 (1960); Kovacic et al., Chem. Rev. 87, 357 (1987); and Brown et al., J. Polym Sci., Polym. Chem. Ed. 24, 255 (1986). This technique, however, results in a mixture of 1,2 and 1,4 units plus chemical defects. Further, the products of this reaction are more properly defined as oligomers rather than polymers as the chain lengths are between 10 and 15 phenylene residues. Moreover, it is difficult to completely remove all of the CuCl.sub.2.
Yamamoto et al., Bull. Chem. Soc. Jpn. 51, 2091 (1978) disclose the polymerization of p-dibromobenzene in the presence of magnesium using a nickel catalyst. However, molecular weight measurements indicate that the growth does not go beyond 10-12 phenylene residues because the polymer separates as a crystalline solid. Thus, further polymerization to a higher molecular weight is difficult or impossible.
Marvel et al., J. Am. Chem. Soc. 41, 448 (1959) disclose a process using cyclohexa-1,3-diene in the presence of a Zieglar catalyst. This produces poly(cyclohexene) containing 1,4 and 1,2 unit+s. Aromatization of the polymers was attempted by reaction with bromine followed by pyrolysis to eliminate hydrogen bromide (HBr). The purity of the products are suspect, however, as there are a number of possible bromine-substituted intermediates which would prevent complete aromatization. Additionally, various attempts to produce pure polyphenylene have tended to fail because of the poor solubility of the partially aromatized product. Moreover, the conditions of the aromatization can be severe enough so that the main polymer chain is fractured.
Ballard et al., Macromolecules 21, 294-304 (1988) describe the synthesis of 5,6-cis-dihydroxycyclohexa-1,3-diene, the polymerization of its derivatives and the conversion of the polymers formed into polyphenylene. In this reaction, the polyphenylene precursor is prepared from benzene using an organic route employing enzymes to convert benzene to a 1,2-disubstituted-3,5-diene. Polymerization and aromatization is completed by the addition of a radical initiator, such as benzoyl peroxide or azobis(isobutyronitrile) ("AIBN"), and subsequent heating of the solution.
U.S. Pat. No. 4,798,742 discloses a multistep process for preparing polyphenylene wherein a copolycyclohexa-3,4-diene is treated under basic conditions to partially aromatize the polymer and then treated to complete aromatization using heat or ultraviolet light.
Despite the variety of methods available to produce polyphenylene, these techniques can result in an oligomer or low mass polymer. Further, the prior techniques can result in impurities in the final product. The processes are also often complex, requiring multiple steps and harsh solvents, and requiring subsequent processing steps after the polymer is formed.